Communication systems generally include transmission of multiple carrier signals in which each of the carrier signals is transmitted at a different transmit frequency. Each individual transmission signal is typically modulated by an information signal. Each of the transmission signals can be individually received, and the information signals can be detected.
FIG. 1 shows a frequency spectrum of multiple transmission signals. The transmission signals each include a carrier frequency FC1, FC2, FC3, FC4. The frequency spectrum allocated to each of the carrier frequencies is generally referred to as a transmission channel. The amount of frequency spectrum allocated to each transmission channel generally determines the amount of information that can be transmitted through the transmission channel. It is desirable to utilize as much of the allocated frequency spectrum as possible.
The frequency spectrum of FIG. 1 shows transmission signals 110, 120, 130, 140 at the carrier frequencies FC1, FC2, FC3, FC4. Frequency spectrum adjacent to each of the transmission signals 110, 120, 130, 140 is generally occupied by information that is modulated onto the transmission signals 110, 120, 130, 140. Generally, the greater the modulation rate of the information (typically, the modulation rate is proportional to the amount of information) the greater the amount of frequency spectrum occupied by each transmission signal and associated modulation information. The modulation rate of each transmission signal should not be so large that the modulation information of one transmission signal interferes with the modulation information of a neighboring transmission signal.
FIG. 2 shows a frequency spectrum of multiple transmission signals 210, 220, 230, 240 in which information from neighboring transmission channels overlap. That is, information intended for transmission through one transmission channel, is unintentionally transmitted within another transmission channel. For example, the modulation information of the first transmission signal 210 overlaps with the modulation information of the second transmission signal 220, as designated 215. The modulation information of the second transmission signal 220 overlaps with the modulation information of the third transmission signal 230, as designated 225. The modulation information of the third transmission signal 230 overlaps with the modulation information of the fourth transmission signal 240, as designated 235.
The overlap can be due to distortion of the transmission signals due to components within a transmission system being non-ideal. The distortion can include noise, spurious signals and harmonics of transmission signals overlapping with neighboring transmission signals.
Information signal channel frequency overlap from one transmission channel to another transmission channel, introduces transmission errors. Transmission errors reduce the effectiveness of a communication system. Additionally, transmission errors can reduce the transmission bandwidth of a communication.
It is desirable to have a method and apparatus that provides selective filtering of multiple frequency signals. It is desirable to provide high frequency filtering of communication signals to reduce the amount of frequency spectrum overlap between transmission signals of the communication signals.